The present invention relates to an inclination monitoring system, which is used, for example, in a lens inclination adjustment apparatus.
Conventionally, a data recording/reproducing device using an optical disk as a recording medium has been known. Such a device is provided with an optical system which includes a light source and an objective lens held in a lens holder. A recording medium (i.e., an optical disk) is set at a focal point of the objective lens. The light source emits a parallel light beam which is focused by the objective lens to form a beam spot on the recording medium. The beam reflects from the recording medium, and is received by an optical pick-up device.
In the data recording/reproducing device, an optical axis of the objective lens preferably intersects the recording medium at right angles. It is because, if the optical axis inclines with respect to a normal line to the recording medium, coma occurs, which may cause the beam spot formed on the recording medium to be wider than it should be. If the beam spot is wider, a density at which recording to the recording medium takes place will be lower. Further, if the beam spot is wider, a reproduced signal may include noise.
Recently, a numerical aperture NA of the optical system for such a device has become larger. In the device which has an optical system having relatively large numerical aperture NA, even a small inclination of the objective lens may affect the size of the beam spot greatly.
Accordingly, it is necessary to adjust the inclination of the object lens such that an inclination angle formed between the optical axis of the objective lens and a line normal to the recording medium is within a permissible range. The permissible range is, for example, a range within 3 minutes.
In order to adjust the inclination of the objective lens, conventionally, a lens inclination adjustment system is used. The conventional inclination adjustment system includes an interferometer unit and a lens inclination adjustment unit.
Firstly, at least a part of the data recording/reproducing device including the optical system and the light source is coupled with the interferometer unit, and interference fringes are observed. A user may determine the inclination angle and inclination direction which is a direction of inclination on a plane parallel to the surface of the recording medium or the cover glass facing the objective lens, based on the observed interference fringes.
Next, the data recording/reproducing device is coupled with the lens inclination adjustment unit, and the lens is moved so that the inclination (i.e., the inclination angle and inclination direction) is adjusted to be in a permissible range in accordance with the inclination angle and the inclination direction determined by the user.
When the lens is moved (i.e., when the inclination status of the lens is changed), it is necessary to monitor the change of the inclination, or current inclination status of the lens. An example of an inclination monitoring system employed in a inclination adjustment apparatus is shown in FIG. 9.
Generally, the objective lens 401 is formed to have a lens portion 402 and a planar flange portion 403 surrounding the lens portion. Monitoring of the inclination status of the lens 401 is performed by editing a beam to the flange portion 403 and detecting the reflected beam.
As shown in FIG. 9, the monitoring system includes an He-Ne laser source 404 and a screen 407. The He-Ne laser source 404 emits a narrow light beam which has a diameter of 1-2 mm towards the flange portion 403 of the lens 401 which is located about one meter away from the He-Ne laser source 404 through an opening 409 formed on the screen 407. The beam reflected by the flange portion 403 is incident on the screen 407. It should be noted that the flange portion 403 may be formed to be a mirror surface.
The reflected light beam is observed as an image on the screen 407, and a center of the image is regarded as a point where the optical axis of the reflected beam intersects the screen 407. The inclination status of the lens 401 is then monitored with reference to the position of the center of the image formed by the reflected beam on the screen 407.
However, since a surface of the flat portion 403 is microscopically uneven as the objective lens is made from a mold, the image of the reflected beam is not a perfect beam spot. As shown in FIG. 10, the image extends over 26 minutes arc in this example, and accordingly the center of the image of the reflected beam is difficult to be identified especially when the inclination status of the lens is being changed.
Therefore, even if the current inclination status is precisely determined with use of the interferometer, it may be difficult to adjust the inclination status since the inclination status cannot be monitored precisely.